Introduction
Biomass is the most abundant renewable resources on earth, and is
considered to be a promising substitute for fossil resources due to its
vast concentration of carbon and hydrogen.1 As the
second largest component of biomass, lignin is presently underutilized
due to its structural complexity, poor solubility and high bond
dissociation energy.2 Conversely, cellulose and
hemicellulose are used widely for the production of bio-ethanol and
sugars industrially.3 At present, 50-70 million tons
of lignin is produced annually from pulping activities, but only
~2% of it is commercially utilized as value-added
materials, such as water-reducing admixtures or as a surfactant,etc .4 On the other hand, it is regarded as the
only renewable source of key- and high-volume aromatic
polymers, making it a potential
green precursor for the production of aromatic products usually refined
from petroleum.5,6 Therefore, the development of
efficient technologies for the production of value-added chemicalsvia lignin depolymerization is not only environmentally benign,
but also meets the requirements for sustainability. To the best of our
knowledge, numerous and different strategies have emerged in recent
years. For example, the addition of formic acid significantly promotes
the yield of aromatics during the depolymerization of oxidized lignin,
giving rise to a value of the yield over 60wt. %.7Moreover, treatment of lignin with formaldehyde protects side-chain
hydroxyl groups, allowing for near-theoretical yields of aromatic
products by hydrogenolysis.8 Recently, the selective
production of diethyl maleate (DEM) was achieved by using
polyoxometalate ionic liquid catalysts (POM ILs) to promote the
selective oxidation of lignin coupled with esterification of the
resulting aromatic monomers.9 Nevertheless, these
processes still exhibit some drawbacks, for instance, high temperature,
high H2 pressure and long reaction time plague the
heterogeneously catalyzed processes to realize both high lignin
conversion and product yields, largely due to the poor contact between
the macromolecule and catalyst. On the other hand, product separation
and isolation can prove quite energy consuming and costly for systems
employing homogeneous catalysts.10 Generally,
process
intensification and coupling techniques are a potential alternative to
solve these problems,11,12 among which the emulsion
approach has received a resurgence in interest recently. Because the
emulsion system is able to provide a much larger interface for reactions
involving in problem related with the incompatibility of
reactants.13
As described above, lignin can be employed as a
surfactant,4,14,15 or as a precursor for the
production of functional surfactants due to the numerous
hydrophilic and hydrophobic groups it
contains.16 Hence, developing an emulsion system
utilizing lignin as the surfactant has obvious advantages, and several
new emulsion systems for lignin depolymerization have been demonstrated
on the basis of its self-surfactivity. For example, over three times of
phenolic monomers’ yield has been obtained in a water/oil (W/O) emulsion
reactor comparing to those in a standard solvent
system.17 Basing on this work, a purpose-designed
emulsion was utilized for the depolymerization of lignosulfonate to
result in the generation of appreciable yields of phenolic monomers and
4-ethyl guaiacol, 116.1 and 39.3 mg g-1respectively.18 Furthermore, Wessling et al.proposed an emulsion system comprised of a deep eutectic solvent and an
extractant for electrochemical oxidation of kraft lignin to low
molecular weight products (ranging from 100 to 600
Da).19 The above examples make it clear that emulsion
approaches can promote lignin depolymerization to some degree, yet
higher lignin conversion facilitates a destabilization of the system and
thus reduces process efficiency, although the demulsification can be
seen as a kind of benefit due to the partitioning effect achieved after
reaction.
To overcome the above difficulties, a surfactant-free microemulsion
(SFME) system could be a better choice for the intensification of lignin
depolymerization, due to its thermodynamic stability and much more
larger interfacial area.20 The SFME is more commonly
referred to as a pre-Ouzo or detergentless microemulsion, so-called due
to the spontaneous formation of a stable emulsion with only the addition
of water, a property that is shared by Ouzo, a famous alcoholic beverage
in Greece.21,22 Thus, SFMEs are known to have
properties similar to those of surfactant-based microemulsions (SBMEs),
providing both the solubilization effect and capacity to dissolve the
immiscibility solvents,23 but not need to separate the
surfactant from the system, showing great advantage both in cost and
process.24,25 Herein, a novel oil/water (O/W) SFME
containing octane, n -propanol and water was proposed after
carefully screening, and the oxidative depolymerization of lignin in
this SFME was conducted basing on the construction of the ternary
diagram and the determination of lignin solubility distribution in its
different subregions. Experimental results showed that around 40 to 500wt. % increase of phenolic monomers in SFME reactor was achieved
with comparison to those in non-microemulsion systems, illustrating
great potential to develop novel SFME for the valorization of biomass.